Pyridoxal 5' phosphate has been found to be a very effective inhibitor of DNA synthesis in vitro catalysed by a variety of DNA polymerases. The inhibition has been characterized as competitive type and probably involves lysine or arginine residue situated at the active site of these enzymes. This discovery then offers the prospect of labelling the active site and permits one to dissect the fine structure of that site so that mechanics of polymerization and enzymatic specificity expressed through such mechanisms may be clearly defined. We have chosen an RNA-directed DNA-polymerase (RLV reverse transcriptase), a DNA dependent DNA polymerase (E. coli DNA Pol. I) and terminal transferase (TdT) from calf thymus for these studies due to their ready availability and current biological interest. The in vivo studies are extensions of in vitro observation and are warranted by the fact that Pyridoxal phosphate (PyP) is a naturally occurring co-factor (co-enzyme) in all living cells and could play a role in control of DNA synthesis. This possibility will be investigated through the comparative study of PyP and levels of DNA polymerases through the cell cycle using synchronized HeLa cells. An additional study towards this end will also be carried out using a prokaryotic system, namely E. coli, which will utilize toluene-permeabilized E. coli cells. The effect of PyP on the DNA synthesis in these permeabilized cells will be investigated.